Multilayered, surface-structured semi-finished product consisting of thermoplastics and film-structuring method

ABSTRACT

The invention relates to a multi-layered semi-finished product with at least one first and one second film layer. The semi-finished product has a velour-type structure on the outer surface of the first film layer facing away from the second film layer and the second layer has a filler material. The first and second film layers are permeable. A first proportion of a filler material in the first film layer is smaller than a second proportion of a filler in the second film layer.

The invention relates to a multi-layer semi-finished product with atleast one first- and one second film layer, wherein the semi-finishedproduct exhibits a velour-like structure at an outer surface facing awayfrom the second film layer and the second film layer has a fillermaterial.

EP 309 073 A2 discloses a method for manufacturing vapour-permeablefilms which consist of drawn polyolefin film with a laminated non-wovenHDPE fabric; the polyolefin film is filled with a high fraction ofinorganic filler so that a water vapour permeability of at least 1000g/m² per day is obtained. In addition to the actual manufacture of thepolyolefin film, the film manufactured by the known method also requiresthe additional process of laminating the non-woven fabric using anadhesive. Especially if the non-woven fabric layer is very thin,problems are encountered in the application of the adhesive. Recyclingcompatibility is difficult to achieve and the required binding time forthe adhesive delays the procedure.

A method for manufacturing a surface-structured, film-like semi-finishedproduct from a thermoplastic material is inherently known from DE 19812097 C1. According to this patent specification, a thermoplastic plasticmaterial in the molten state or in the form of a film is applied to asurface with fine cavities constructed as a negative structure (matrix)in relation to the desired structure. The matrix surface is exposed toan under-pressure from outside or from inside so that the cavitiesbelonging to the matrix are evacuated. The matrix including the cavitiesis at least partly filled with the thermoplastic plastic material andthe deformed thermoplastic plastic material, still lying on the surface,is made to solidify by cooling, wherein it takes on the correspondingsurface structure on the side brought in contact with the surface. Aftersolidifying, the plastic material is removed from the surface whereinthe thermoplastic material which has penetrated into the cavities andbeen withdrawn from these forms a pile consisting of projections.

The projections forming the pile are drawn by combing, brushing,scraping and/or shear mangling wherein the length of the projections isincreased by at least twice the original length and a fibre-likestructured semi-finished product is formed at least on one side wherethe projections are stretched to form hair fibres.

Tests have shown that it is not sufficient to change the formulation ofsaid film in the aforesaid patent specification such that a filler of aninherently known type of organic or inorganic origin is mixed with saidfilm to make the film into a porous or vapour-permeable film afterdrawing. If the film material is filled very high, that is, as muchfiller is added as would be required to produce the vapour permeability,the film surface or the melt can then no longer be made sufficientlyfluid to produce the fine hair fibres.

With a two-layer film there is basically the problem that a highlyfilled mass indeed produces a film in the drawn state having sufficientvapour permeability but the second layer for which a melt suitable forhair formation is used according to the prior art, again cannot be drawnto produce vapour permeability.

The object is thus to provide a multi-layer semi-finished product of thetype specified initially which is permeable to vapour and/or liquid.

This object is achieved with a multi-layer semi-finished product with atleast one first and one second film layer, wherein the semi-finishedproduct exhibits a velour-like structure at an outer surface of thefirst film layer facing away from the second film layer and the secondfilm layer has a filler material, by the fact that the first and thesecond film layers are permeable, wherein a first fraction of a fillermaterial in the first film layer is smaller than a second fraction of afiller material in the second film layer.

A suitable velour film is accordingly obtained if a polymer material isprepared and melted in at least two extruders, each polymer preparationis supplied to a nozzle and the melt emerging from the nozzle is broughtin direct contact with a matrix roller. For example, a matrix rollerhaving 1500 to 10,000 holes per square centimeter with a depth of 250 to450 μm and a hole diameter of 40 to 80 μm has proved to be suitablehere.

Preferably suitable as the matrix surface is a steel roller having fineholes of said dimensions. Steel rollers having a plastic surface canalso be used. Fine holes of said type can be produced using a laser.

The melt applied to the matrix surface can be incorporated more easilyinto the cavities if the cavities, as is inherently known, are madealmost empty of air by preliminary evacuation or by applying a vacuum tothe bottom of the cavities so that the melt is sucked into the cavities.After cooling the melt solidifies and a film can be removed from thematrix. First produced is a two-layer film having a plurality of finenaps on its inner surface. The naps can be drawn by suitable andinherently known treatment, especially combing, brushing, scrapingand/or shear mangling.

In another method a film can be manufactured using a film which hasalready been pre-produced. In this case, a forming tool is used for thefilm, which consists of at least two rollers, namely a heated steelroller and as the second roller, a steel roller provided with a plasticcladding in which the matrix with the cavities has been produced by alaser beam.

In this case also, the matrix has between 200 and 10,000 extremely smallholes per square centimeter with a hole diameter of 50 to 80 μm and ahole depth of approximately 600 μm. The film to be processed is heatedin the roller slot or just before it so that said film begins to flow atits surface. The polymer layer at the front of the film is pressed intothe cavities under the pressure in the roller slot and is pressed out asa result of the overpressure when removing the film.

After cooling a fine nap structure appears on the film. This can beenlarged by at least twice the original length by combing, brushing,scraping and/or shear mangling so that the projections are lengthened toform hair fibres.

The object is thus to provide an improved method where the aforesaiddisadvantages do not occur and with which it is possible to manufacturea multi-layer, surface-structured, porous, film-like, semi-finishedproduct from thermoplastic plastics without changing the proceduralunits. The fields of application of these semi-finished products shouldlie in the fields of domestic products, hygiene products and otherfields of application where porous or vapour-permeable films orsemi-finished products are required.

The object is achieved by a film structuring method of the typespecified initially, having the features of claim 9.

In the new method two formulations are used for the melts. The substratefilm not equipped with velour contains a larger fraction of fillers thanthe front of the film equipped with velour.

Calcium carbonate or barium sulphate mixed with the substrate film in afraction between 40 and 50 wt. % are used as fillers, for example,whereas the film layer at the front side only contains 20 to 30 wt. % ofthe same filler. The finely ground inorganic fillers having a grain sizeof preferably 3 to 5 μm can be mixed with water-repelling substances orsurfactants such as calcium stearate which facilitate working into theplastic mass.

Suitable as fillers are organic and/or inorganic substances having lowaffinity to the surrounding thermoplastic plastic and a significantlylower elasticity than said plastic. Fillers can be selected from thegroup comprising calcium carbonate, talc, clay, kaolin, quartz,diatomaceous earth, magnesium carbonate, barium carbonate, magnesiumsulphate, barium sulphate, calcium sulphate, zinc oxide, magnesiumoxide, titanium dioxide, glass powder or zeolite.

Preferably used as polymers for manufacturing the films are polyolefinssuch as polyethylene or polypropylene, mixtures of said polymers,copolymers and mixtures of homopolymers and copolymers. However, it isalso possible to use plastics based on other thermoplastics such aspolyesters, copolyesters, polyamides, polyether esters, polyetheramines, polyvinyl alcohols, polyvinyl alkanols, and mixtures orcopolymers of said polymer groups.

In addition to a first and a second film, several films lying one abovethe other can also be used. Each film can consist of a differentmaterial, for example, metal film or others. Each film can have aspecial function: stabilisation, insulation, thermal and/or electricalconduction, coloration, identification etc.

By means of a subsequent drawing process with a suitable mixture, thetwo-layer film is given both a velour surface and water vapourpermeability. The film is stretched in the direction of the machine orin both directions if pre-manufactured blown film is used. In the caseof cast films or films cooled directly on the matrix, stretching ispreferably in the transverse direction to the machine running direction.In order to achieve stretching in the machine running direction, afterlengthening the hairs the velour film is passed over two rollers whichrun at different speeds. The second roller is driven somewhat faster andthus pulls the film in the machine running direction so that it isstretched.

In order to achieve stretching transverse to the machine runningdirection, stretching in a stretching frame is used. Another type ofstretching uses rollers which partially stretch the films in a left andright ascending fashion with intermeshing elements. Here one roller isheld fixed in its bearing and driven. The counter-roller preferablypenetrates at least 2 mm into the fixed roller, depending on therequired degree of stretching. The depth of penetration of the roller isvariable. The teeth of the upper roller are driven until they stall inorder to prevent damage to the teeth of the lower roller. It is notnecessary to drive the upper roller since power is transmitted throughthe film to be stretched.

Another roller configuration consists of a pair of rollers havingleftward and rightward milled slots which intermesh in a cogwheelfashion. The milled slots are rounded on their outer side and have veryfinely worked webs approximately 0.7 mm thick. Both rollers have adiameter of 200 to 250 mm. The penetration depth is a maximum of 3 to 6mm, with a working depth of approximately 2 to 2.5 mm being preferred.

A second pair of rollers provides for CD stretching. Here an offset ofaround 90° is used compared with said apparatus. The roller diameter isalso 200 to 250 mm. The web thickness is 0.78 mm. The maximumpenetration depth is 6 mm. For the film material there is a clearance ofapproximately 0.4 to 0.5 mm on both sides of the film track. Dependingon the stretching, the penetration depth during the forming is around 2to 2.5 mm. Stretching takes place in all cases at normal roomtemperature, that is approximately between 25 and 30° C. The performancein metres per minute is comparatively very high.

The film becomes broader and thinner as a result of the stretching,i.e., an approximately 80 μm thick polyethylene film having a squaremeter weight of 80 gram will weigh about 55 to 60 gram per square meterafter stretching. A transversely stretched film having an initialthickness of around 60 μm weighs around 42 g per square meter afterstretching. The water vapour permeability is 1500 to 3000 g/m² daydepending on the degree of stretching, measured in accordance with ASTME 96 E, at a measurement temperature of 37° C.

The teeth of the rollers can also be operated offset at a particularangle or perpendicular to intake or running. The pattern of the film canbe varied accordingly within the play of the rollers and the stretchingthus produced.

It should be emphasised that the textile feel of the velour film isbarely changed by the stretching process since the code density of thenaps present on the film surface keep the appearance and the feel of thefilm almost unchanged even after stretching.

As a result of stretching both in the machine direction and transversethereto, a geometric picture of a fabric-like warp and weft structure isobtained visually for the film. If diagonal drawing is used, thefinished film has an interesting satin-like appearance.

AN EXAMPLE IS USED TO EXPLAIN THE METHOD

A double-nozzle extrusion plant is used to produce a two-layerpolyethylene film having a front layer made of relatively light flowingpolyethylene and a back layer made of a more viscous, less flowingpolyethylene. Both layers are filled with calcium carbonate having anaverage grain size of 1.0 μm which was surface-treated with calciumstearate. The polyethylene mass of the front layer contains preferably20 to 30 wt. % of a filler material, especially calcium carbonatewhereas the back layer preferably contains at least 50 wt. % of a fillermaterial, especially calcium carbonate as filer.

Both film layers have the same thickness; the total thickness is 60 μmto 80 μm.

For the front layer a mixture of 70 parts of a polyethylene having amelting index of 30, measured in accordance with ASTM D 1238, and havinga density of 0.885 g/cm³ measured in accordance with ASTM D 792, wasmixed with 30 parts of a polyethylene resin, especially anethylene-octene copolymer, having a melting index of 30 deg/min and adensity of 0.902 g/cm² in accordance with ASTM D 792, 22 wt. % ofsurface-treated calcium stearate as well as known stabilisers andpigments, and pre-granulated. This polymer mixture was then melted in anextruder and supplied to the double nozzle.

A second extruder was loaded with a HDD polyethylene containing 45 wt. %calcium carbonate and having a melting index of 2.1 deg/min and adensity of 0.920 g/m² before the calcium carbonate was added.

The two-layer film is then produced by the chill roll method. The cooledfilm is converted into a velour film using forming tools. A polyethylenefilm with a distinct velour character is thus produced. This film isstretched by 30 to 50% in the machine direction and by 25 to 60% in thetransverse direction thereto so it acquires a fine microporosity. Afterstretching at a temperature of 25 to 30° C. the velour film has a watervapour permeability of 2500 g/m² day measured in accordance with ASTM E96 E.

1. A multi-layer semi-finished product with at least one firstthermoplastic and one second thermoplastic film layer, wherein thesemi-finished product exhibits a velour structure at an outer surface ofthe first film layer facing away from the second film layer and thesecond film layer has a filler material, characterized in, that thefirst thermoplastic and the second thermoplastic film layers arepermeable, wherein a first fraction of a filler material in the firstthermoplastic film layer is smaller than a second fraction of a fillermaterial in the second thermoplastic film layers, and wherein the firstand second thermoplastic film layers contain filler material.
 2. Thesemi-finished product according to claim 1, characterized in that thefirst thermoplastic and the second thermoplastic film layers are atleast partly permeable to vapour and impermeable to liquid.
 3. Thesemi-finished product according to claim 1, characterized in that thefirst thermoplastic film layer has a first fraction of filler materialof up to 30 percent by weight.
 4. The semi-finished product according toclaim 1, characterized in that the first thermoplastic and the secondthermoplastic film layers have the same filler material.
 5. Thesemi-finished product according to claim 1, characterized in that afirst material of the first thermoplastic film layer is slightly lessflowing that a second material of the second thermoplastic film layer.6. The semi-finished product according to claim 1, characterized in thatthe semi-finished product has a water vapour permeability between 1500and 3000 g/m²·day.
 7. The semi-finished product according to claim 1,characterized in that the filler material is inorganic with asurfactant.
 8. The semi-finished product according to claim 1,characterized in that the semi-finished product weighs approximately 60g/m² and less.
 9. The semi-finished product according to claim 1,characterized in that the filler material is calcium carbonate that issurface-treated with calcium stearate.
 10. A film structuring methodcomprising: providing of at least one first thermoplastic and one secondthermoplastic film layer, wherein the first thermoplastic film layer hasa smaller fraction of filler material than the second thermoplastic filmlayer, and wherein the first and second thermoplastic film layerscontain filler material, producing of a surface-structured film-likesemi-finished product from a surface of the first thermoplastic filmlayer forming an outer surface of the semi-finished product, wherein avelour structure is worked using a forming tool.
 11. The filmstructuring method according to claim 10, characterized in that thefirst thermoplastic and the second thermoplastic film layers arestretched separately one from the other.
 12. The film structuring methodaccording to claim 10, characterized in that the semi-finished productis stretched between 30% and 50% in the machine direction and between25% and 60% in the transverse direction.
 13. The film structuring methodaccording to claim 10, characterized in that the semi-finished productis stretched diagonally.
 14. The film structuring method according toclaim 10, characterized in that the velour structure is lengthened. 15.The film structuring method according to claim 10, characterized in thatthe semi-finished product is stretched so that a vapour-permeable andwater-impermeable semi-finished product is produced.
 16. Afilm-producing apparatus to produce a semi-finished product with avelour surface structure according to claim 10, wherein the apparatushas: a feed apparatus for at least one first and one secondthermoplastic film layer filled with filler material to supply theselayers lying one above the other to an adjacent processing station, aforming tool to which the film layers lying one above the other can beapplied, a stretching device to stretch the semi-finished product toproduce a permeable semi-finished product, wherein the stretching deviceis arranged so that a semi-finished product with a velour outer surfaceis supplied to it.
 17. The film structuring method according to claim10, characterized in that the first and second film layers are stretchedbefore or after working the structure to obtain a permeablesemi-finished product.